State of charge (SOC) monitoring is desirable or necessary in many battery applications, including portable electronics products such as wireless communications devices and laptop computers, power tools, electric vehicles (including hybrid, plug-in hybrid, and all-electric vehicles), backup power systems, energy storage for power generation devices such as solar or wind collectors or fuel cells or conventional fuel-burning power sources, and the like. A battery, or string of batteries forming a battery pack, may be used over a limited range of SOC or over a wide range including the entire capacity available from the battery.
Accurate knowledge of the state of charge (SOC) and state of health (SOH) of a battery is important for many applications, and especially so for long-life, high charge or high discharge rate applications such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (EVs). In hybrid electric vehicles, it is especially desirable to monitor the state of charge of the battery since operation does not typically use the whole range of SOC and typically uses an SOC range that is centered around approximately 50% SOC, for example, within approximately 10-90% or 40-60% of SOC. Monitoring SOC and SOH can be difficult if the voltage of the battery varies relatively little with the SOC, or if the voltage is time-dependent at a constant SOC, or if hysteresis of voltage occurs so that the cell voltage is charge/discharge history-dependent.
There are a number of situations in which it is desirable to know the potential of each electrode in an electrochemical cell with some accuracy. The potential at any one electrode in a battery may undergo excursions in normal operation that brings it close to a potential that can cause damage or degrade performance or life. For example, there may be too high a potential at the positive electrode causing electrolyte degradation or an overcharged positive active material. In the case of a lithium ion battery, the potential may be too low at the negative electrode causing lithium metal plating.
As another specific example wherein detailed knowledge of the electrode potential is needed in a practical battery, consider a lithium ion battery undergoing charging at high rates. Too high a charge rate or degradation of the cell can cause the potential at the negative electrode to drop below that of lithium metal and cause lithium plating at the negative electrode, which degrades life and can create a safety concern. However, if the potential at the negative electrode were known with accuracy, a battery management system could be designed to cease charging of the cell before significant lithium plating occurs.
Another reason to monitor SOC accurately is to improve the life or safety of the battery. Some battery chemistries become unsafe at too high a charge voltage, and many chemistries degrade faster at very high or very low SOC. An accurate SOC estimate is therefore useful for optimizing the system for safety or long life.
Therefore, it may be critical to know with accuracy the potential at each electrode. However, the cell voltage, while easily measured, gives the difference in potential rather than the absolute potential, and includes various polarization contributions which may differ in magnitude between the positive and negative electrode, thereby making determination of the electrode potential difficult. New performance demands such as an HEV have created a need for better SOC/SOH monitoring. Existing reference electrodes such as lithium metal may not be suitable for lithium ion battery systems used under the above-described demanding conditions due to insufficient stability and life (e.g. drift of the reference potential) or unsuitable reference potential.